Opamp Guard Polygon: Only in Top PCB layer or or all layers

Question

I have an opamp circuit as shown below.

My board stack up is SIGNAL-GND-POWER-SIGNAL.

I am providing a polygon to Guard the trace which is going to non-inverting the input of the opamp.

The guard trace is connected to SMA connector also. At present polygon is provided only in the 1st layer (SIGNAL) of PCB

My question is:

Do I need to place the guard polygon in other layers also, or can I keep other layers as it is?

Please see the layout below.

The yellow color is the signal that needs to be guarded. The red color is the polygon guard ring. May I know is this fine or not. The Guard ring is not fully enclosing the Non inverting input pin of opamp,

Updated layout

Answer 1

A guard is not the same as a grounded shield. In this case, the guard is at ROUGHLY the same voltage as your signal. The point of the guard is to prevent current leakage across the various insulators. SO, you'll need to calculate how much leakage you'll have in the Z-axis from the PCB material before you decide whether or not to add the GUARD poly to the layer below. In any case, you're only talking about adding the guard on the very next layer down (adding on every layer will have diminishing returns).

For more information, a very helpful source is a Keithley book "Low level measurements handbook". You can find it here: https://www.tek.com/en/documents/product-article/keithley-low-level-measurements-handbook---7th-edition

Read Chapter 2 for "GUARD"ing information.

Answer 2

If you want to minimize the electrical leakage and coupling to other circuit nodes, then.

1. Add a guard trace encircling the high impedance node.

2. Add closely spaced vias through the guard trace.

3. Add a polygon on a layer directly below the guard trace, with the guard trace's projection onto the underlying layer being the outline of the polygon.

4. Leave other underlying layers either empty, or use one of them as an electrostatic shield connected to GND.

Essentially, the high-impedance node should be floating in insulating material (FR-4), and the "tub" filled with the insulator should be like a metal bathtub, at the same potential as the high-impedance node, but should be low-impedance (i.e. buffered).

Answer 3

Your GND plane should already provide shielding under your components, providing it is a continuous ground plane. The top layer calls for a guard ring if it's advised by the manufacturer. Keep traces short and straight from source to destination if possible ...